Controlling the Electromagnetic Properties of Magnetic Composites and Metamaterials

Ferrites are a class of magnetic oxides with superior electromagnetic (EM) properties at microwave frequencies when compared to conventional metallic magnetic materials for use in antenna miniturization and radar absorbers. Metamaterials are also a special group of materials, which are known to provide EM responses not found in nature due to subwavelength structuring. In this thesis, a range of ferrite composite materials and metamaterial structures are exploited to develop new methods for controlling permittivity and permeability up to 4 GHz with a view to producing high refractive index materials and to demonstrate broadband impedance matching to free space. The rst section of the thesis uses composites of powdered MnZn ferrite (as the ller) and PTFE (as the matrix), fabricated by a novel cold pressing technique, to produce composites for a range of volume fractions of MnZn ferrite (between 0-80% vol.). The EM properties for all composites were determined as a function of % vol. and the results were found to be in agreement with the Lichtenecker mixing formula. This study is the rst convincing con rmation of the Lichtenecker mixing formula over a broad range of volume fractions (0-80% vol.). The cold pressing method was found to produce composites with reproducible EM properties, and was extended to use aluminium, barium titanate (as llers) and also cellulose as an alternative matrix. Importantly, with regard to the study of cellulose composites, our work is the rst to explore volume fractions of up to 85% and, the rst to con rm the Lichtenecker mixing formula with these materials. The ferrite particle size, as well as the volume fraction of ferrite, impacts the EM response of the composites. Both the permittivity and permeability increase as a function of ferrite particle size; however, the permeability increases at a much faster rate than the permittivity with particle size. It is shown that by controlling the ferrite particle size in conjunction with the volume fraction of ferrite, broadband impedance matching to free space can be realised for tailored values of refractive index. This is the rst study that demonstrates independent control of the permittivity and permeability of ferrite composites by controlling the ferrite particle size. Alternatively, by adding a third component to the two part composite it is demonstrated that broadband impedance matching to free space can also be realised with a refractive index of 16.1 (between 10-50 MHz). This is the rst time, to the authors knowledge, that three part composites have been used to achieve high refractive index materials that are impedance matched to free space. The second section of this thesis takes the concept of metamaterials to structure ferrite composite material with a further view to gain independent control over the permittivity and permeability. By tailoring the EM response of this metamaterial, which is comprised of anisotropic arrays of ferrite cubes, broadband impedance matching to free space is demonstrated. The refractive index over the impedance-matched frequency range is also very high (9.5). The metamaterial also acts as an excellent non-re ecting subwavelength thickness absorber up to 200 MHz. An analytical description of the permittivity and permeability dependence on the metamaterial parameters is developed to predict the EM response of this metamaterial, and of similar systems. In the last part of this thesis, the concept of cubic metamaterials is extended to more complex metallic meta-atoms, where the permittivity and diamagnetic response of the metamaterial are independently tailored to demonstrate how the refractive index can be tuned over a broad frequency range. By understanding the role of individual cube parameters, the diamagnetic response can be controlled between near zero and unity, which greatly alters the refractive index. The results are the rst experimental validation for showing `design' control of the permittivity and permeability of these metamaterials via geometry tuning of the meta-atom design.EPSRC, DST

Heavily loaded ferrite-polymer composites to produce high refractive index materials at centimetre wavelengths

A cold-pressing technique has been developed for fabricating composites composed of a polytetrafluoroethylene-polymer matrix and a wide range of volume-fractions of MnZn-ferrite filler (0%–80%). The electromagnetic properties at centimetre wavelengths of all prepared composites exhibited good reproducibility, with the most heavily loaded composites possessing simultaneously high permittivity (180 ± 10) and permeability (23±2). The natural logarithm of both the relative complex permittivity and permeability shows an approximately linear dependence with the volume fraction of ferrite. Thus, this simple method allows for the manufacture of bespoke materials required in the design and construction of devices based on the principles of transformation optics

A Pilot Survey of an M Dwarf Flare Star with Swift's UV Grism

The near-ultraviolet (NUV) spectral region is a useful diagnostic for stellar flare physics and assessing the energy environment of young exoplanets, especially as relates to prebiotic chemistry. We conducted a pilot NUV spectroscopic flare survey of the young M dwarf AU Mic with the Neil Gehrels Swift Observatory's UltraViolet and Optical Telescope. We detected four flares and three other epochs of significantly elevated count rates during the 9.6 hours of total exposure time, consistent with a NUV flare rate of $\sim$0.5 hour$^{-1}$. The largest flare we observed released a minimum energy of 6$\times$10$^{33}$ erg between 1730-5000 \r{A}. All flares had durations longer than the $\sim$14-17 minute duration of each Swift visit, making measuring total flare energy and duration infeasible.Comment: Published in Research Notes of the AAS (RNAAS

Exploring the relationships between psychological variables and loot box engagement, part 1: pre-registered hypotheses

Loot boxes are purchasable randomized rewards in video games that share structural and psychological similarities with gambling. Systematic review evidence has established reproducible associations between loot box purchasing and both problem gambling and problem video gaming, perhaps driven by a range of overlapping psychological processes (e.g. impulsivity, gambling-related cognitions, etc.) It has also been argued that loot box engagement may have negative influences on player financial and psychological wellbeing. We conducted a pre-registered survey of 1495 loot box purchasing gamers (LB cohort) and 1223 gamers who purchase other, non-randomized game content (nLB cohort). Our survey confirms 15 of our 23 pre-registered hypotheses against our primary outcome (risky loot box engagement), establishing associations with problem gambling, problem gaming, impulsivity, gambling cognitions, experiences of game-related ‘flow’ and specific ‘distraction and compulsion’ motivations for purchase. Results with hypotheses concerning potential harms established that risky loot box engagement was negatively correlated with wellbeing and positively correlated with distress. Overall, results indicate that any risks from loot boxes are liable to disproportionately affect various ‘at risk’ cohorts (e.g. those experiencing problem gambling or video gaming), thereby reiterating calls for policy action on loot boxes

Exploring the relationships between psychological variables and loot box engagement, part 2: exploratory analyses of complex relationships

In a pre-registered survey linked to this paper (Exploring the relationships between psychological variables and loot box engagement, part 1: pre-registered hypotheses), we confirmed bivariate associations between engagement with loot boxes (purchasable randomized rewards in video games) and measures of problem gambling, problem video gaming, impulsivity, gambling cognitions, experiences of game-related ‘flow’, psychological distress and reduced wellbeing. However, these variables have complex relationships, so to gain further insights, we analysed the dataset (1495 gamers who purchase loot boxes and 1223 purchasers of non-randomized content) in a series of Bayesian mixed-effects multiple regressions with a zero-inflation component. The results challenge some well-established results in the literature, including associations between loot box engagement and problematic gambling measures, instead suggesting that this relationship might be underpinned by shared variance with problem video gaming and gambling-related cognitions. An entirely novel discovery revealed a complex interaction between experiences of flow and loot box engagement. Distress and wellbeing are both (somewhat contradictorily) predictive of participants engaging with loot boxes, but neither correlate with increasing loot box risky engagement/spend (among those who engage). Our findings unravel some of the nuances underpinning loot box engagement, yet remain consistent with narratives that policy action on loot boxes will have benefits for harm minimization

Exploring the relationships between psychological variables and loot box engagement, part 1: pre-registered hypotheses

Loot boxes are purchasable randomized rewards in video games that share structural and psychological similarities with gambling. Systematic review evidence has established reproducible associations between loot box purchasing and both problem gambling and problem video gaming, perhaps driven by a range of overlapping psychological processes (e.g. impulsivity, gambling-related cognitions, etc.) It has also been argued that loot box engagement may have negative influences on player financial and psychological wellbeing. We conducted a pre-registered survey of 1495 loot box purchasing gamers (LB cohort) and 1223 gamers who purchase other, non-randomized game content (nLB cohort). Our survey confirms 15 of our 23 pre-registered hypotheses against our primary outcome (risky loot box engagement), establishing associations with problem gambling, problem gaming, impulsivity, gambling cognitions, experiences of game-related ‘flow’ and specific ‘distraction and compulsion’ motivations for purchase. Results with hypotheses concerning potential harms established that risky loot box engagement was negatively correlated with wellbeing and positively correlated with distress. Overall, results indicate that any risks from loot boxes are liable to disproportionately affect various ‘at risk’ cohorts (e.g. those experiencing problem gambling or video gaming), thereby reiterating calls for policy action on loot boxes

Development and Validation of the RAFFLE: A Measure of Reasons and Facilitators for Loot Box Engagement

Qualitative studies have identified a diverse array of motivations for purchasing items within video games through chance-based mechanisms (i.e., “loot boxes”). Given that some individuals—particularly those at risk of disordered gaming and/or gambling—are prone to over-involvement with loot box purchasing, it is important to have a reliable, valid means of measuring the role of different motivations in driving purchasing behaviour. Building on prior qualitative research, this paper reports the development and validation of the “RAFFLE” scale, to measure the Reasons and Facilitators for Loot box Engagement. A 23-item, seven-factor scale was developed through cognitive interviews (n = 25) followed by two surveys of UK-based gamers who purchase loot boxes; analysed via exploratory (n = 503) and confirmatory (n = 1495) factor analysis, respectively. Subscales encompassed “enhancement’; “progression’; “social pressure’; “distraction/compulsion’; “altruism’; “fear of missing out’; and “resale”. The scale showed good criterion and construct validity (correlating well with measures of loot box engagement; the risky loot box index (r = 0.63) and monthly self-reported spend (r = 0.38)), and good internal validity (Cronbach’s alpha = 0.84). Parallels with, and divergence from, motivations for related activities of gaming and gambling, and alignment with broader theoretical models of motivation, are discussed

Fibro-inflammatory recovery and type 2 diabetes remission following a low calorie diet but not exercise training: A secondary analysis of the DIASTOLIC randomised controlled trial

AimsTo investigate the relationship between fibro-inflammatory biomarkers and cardiovascular structure/function in people with Type 2 Diabetes (T2D) compared to healthy controls and the effect of two lifestyle interventions in T2D.MethodsData were derived from the DIASTOLIC randomised controlled trial (RCT) and includes a comparison between those with T2D and the matched healthy volunteers recruited at baseline. Adults with T2D without cardiovascular disease (CVD) were randomized to a 12-week intervention either: (1) exercise training, (2) a low-energy (∼810 kcal/day) meal-replacement plan (MRP) or (3) standard care. Principal Component and Fisher's linear discriminant analysis were used to investigate the relationships between MRI acquired cardiovascular outcomes and fibro-inflammatory biomarkers in cases versus controls and pre- and post-intervention in T2D.ResultsAt baseline, 83 people with T2D (mean age 50.5 ± 6.4; 58% male) and 36 healthy controls (mean age 48.6 ± 6.2; 53% male) were compared and 76 people with T2D completed the RCT for pre- post-analysis. Compared to healthy controls, subjects with T2D had adverse cardiovascular remodelling and a fibro-inflammatory profile (20 differentially expressed biomarkers). The 3D data visualisations showed almost complete separation between healthy controls and those with T2D, and a marked shift towards healthy controls following the MRP (15 biomarkers significantly changed) but not exercise training.ConclusionsFibro-inflammatory pathways and cardiovascular structure/function are adversely altered before the onset of symptomatic CVD in middle-aged adults with T2D. The MRP improved the fibro-inflammatory profile of people with T2D towards a more healthy status. Long-term studies are required to assess whether these changes lead to continued reverse cardiac remodelling and prevent CVD

Clinical characteristics, risk factors and outcomes in patients with severe COVID-19 registered in the International Severe Acute Respiratory and Emerging Infection Consortium WHO clinical characterisation protocol: a prospective, multinational, multicentre, observational study

Due to the large number of patients with severe coronavirus disease 2019 (COVID-19), many were treated outside the traditional walls of the intensive care unit (ICU), and in many cases, by personnel who were not trained in critical care. The clinical characteristics and the relative impact of caring for severe COVID-19 patients outside the ICU is unknown. This was a multinational, multicentre, prospective cohort study embedded in the International Severe Acute Respiratory and Emerging Infection Consortium World Health Organization COVID-19 platform. Severe COVID-19 patients were identified as those admitted to an ICU and/or those treated with one of the following treatments: invasive or noninvasive mechanical ventilation, high-flow nasal cannula, inotropes or vasopressors. A logistic generalised additive model was used to compare clinical outcomes among patients admitted or not to the ICU. A total of 40 440 patients from 43 countries and six continents were included in this analysis. Severe COVID-19 patients were frequently male (62.9%), older adults (median (interquartile range (IQR), 67 (55-78) years), and with at least one comorbidity (63.2%). The overall median (IQR) length of hospital stay was 10 (5-19) days and was longer in patients admitted to an ICU than in those who were cared for outside the ICU (12 (6-23) days versus 8 (4-15) days, p<0.0001). The 28-day fatality ratio was lower in ICU-admitted patients (30.7% (5797 out of 18 831) versus 39.0% (7532 out of 19 295), p<0.0001). Patients admitted to an ICU had a significantly lower probability of death than those who were not (adjusted OR 0.70, 95% CI 0.65-0.75; p<0.0001). Patients with severe COVID-19 admitted to an ICU had significantly lower 28-day fatality ratio than those cared for outside an ICU

Global outbreak research: harmony not hegemony

No abstract available